What causes colored snowfalls?

With all due respect to the Brothers Grimm and Walt Disney, snow is not always snow white. It can be pink, red, green or blue, along with a variety of less common shades. Its color is not the result of a spell that’s been cast, a prince’s magical kiss or a curse stirred up in a witch’s bubbling cauldron. And that’s no fairy tale.

Although it does seem as if it would have supernatural origins, colored snow actually has scientific explanations involving either the properties of light, the traits of lower life forms – or both.

Watermelon snow

Multi-hued snow, colored primarily by numerous algae species, is typically abundant in polar regions throughout the spring and summer, and acquires its color from algae after it has fallen – not before or during a snowstorm. These algae flourish at temperatures less than 50 degrees Fahrenheit, tolerate frost, and can withstand the long-lasting darkness of an Arctic night, or its equally excessive daylight. Through the winter, the algae lie dormant beneath the snow.

During the spring thaw, the algae revive and consume nutrients flowing from thawing boulders, trees, soil and insects. Using this fuel, they thrust themselves to the snow’s surface with tails called flagella. Atop the snowdrift, they lose their tails and squeeze together, creating a banner of bright color.

Nearly 460 species of snow algae have been identified worldwide. One of the most common species, Chlamydomonas nivalis, is linked with a red/pink or green snow, known as watermelon snow for its coloring, along with its sweet smell and taste reminiscent of a watermelon. Watermelon snow is spread worldwide in the alpine regions, the polar regions and at altitudes of 10,000 to 12,000 feet. It, like other snow algae species, grows robustly wherever there is a massive accumulation of snow and ice. Algae-colored snow can be found in Greenland, Antarctica, Alaska, Japan, Europe, China, the Himalayas and the coasts of North America.

For thousands of years, mountain climbers, explorers and naturalists were puzzled by the pink snow. Some people speculated that it was formed by minerals or oxidation leached from rocks. The ancient Greek philosopher Aristotle (384–322 B.C.E.) offered the first known written descriptions of watermelon snow, although he was unable to provide an explanation for the phenomenon: “…living animals are found in substances that are usually supposed to be incapable of putrefaction; for instance, worms are found in long-lying snow; and snow of this description gets reddish in color, and the grub that is engendered in it is red, as might have been expected, and it is also hairy.”

Banks of vibrant pink snow can be seen during the summertime on the alpine slopes of California’s Sierra Nevada. They can be viewed at Yosemite National Park at an elevation of approximately 11,000 feet. In the Western United States’ high mountain ranges, a variety of about 60 species of snow algae have been discovered, but only a few types have been accounted for in the Sierra Nevada.

Unlike the predominant species of fresh-water algae, Chlamydomonas nivalis thrives in the cold and flourishes in freezing water. According to estimates, one teaspoon of melted pink snow can harbor more than one million cells of snow algae. If you press on the pink snow with your boot, or compact it in any other way – such as compressing it into a snowball or collecting it in a jar – you’ll increase the density of the red cells, thus intensifying their red hue.

The bright red – or carotenoid – pigment infusing snow algae cells bears a chemical resemblance to the pigment found in red peppers, tomatoes, brightly colored flowers and autumn leaves. It’s presumed that carotenoid pigments protect snow algae’s delicate cells from fierce solar radiation at the snow’s surface. Alpine snowbanks are only availed of a thin layer of atmosphere for filtration, so they are at the mercy of more damaging ultraviolet radiation than snowbanks at lower elevations.

The algae is equipped with photosynthetic chlorophyll, the substance responsible for its often-green coloring, and its resulting green snow, as well as a secondary red or pink pigment called astaxanthin. This secondary hue is a protectant that shields the green algae from ultraviolet light, according to Ron Hoham, a biology professor at Colgate University, and retains energy to melt snow and supply the algae with liquid water.

Blue snow

First, let’s clear up a misconception. The blue in snow and glaciers is not produced by light scattering. That’s the phenomenon that makes the sky blue. When light bounces off molecules and small dust particles in the atmosphere, it produces a blue sky. However, when light passes through snow or ice, red light is absorbed and blue light is transmitted – no light scattering is involved.

A substantial thickness of snow or ice is necessary for absorbing enough red light so that only the blue emanates. Over and over again, light is passed between snow and ice grains, shedding red bit by bit.

Glaciers appear to have a blue cast because of the repeated bending of light by layers of snow. This causes red light frequencies to be absorbed. When red is extracted from light, the frozen surfaces appear blue.

During its first year, Arctic ice is initially white because it is filled with bubbles. Light only treks a short distance before it’s scattered by the bubbles and ricocheted back out. This leaves little opportunity for absorption to take place, and the light exits with the same color it had when it entered. However, during the summer, the ice surface melts, creating new overlaid ice layers that compress lingering air bubbles. Any light now entering traverses a longer distance inside the ice before it surfaces. This allows the red portion of the spectrum plenty of space to be absorbed, and the light emerging at the surface is blue, tinting the snow or ice blue, as well.

Red, orange and brown snow

Although watermelon snow and other algae-based snow falls white and becomes tinted as algae develops on it, snow may fall red, orange or brown because of dust, sand or airborne pollutants. This aberration famously occurred in 2007 when 580 square miles of Siberia were blanketed in orange-colored snow. Foul-smelling and oily, the area it affected was populated with approximately 27,000 residents. Although speculation regarding its origin wildly ran the gamut from industrial pollution to a rocket launch, and even to a nuclear accident, it was later determined that the orange snow was probably the result of a severe sandstorm in nearby Kazakhstan.

Gray or black snow

Gray or black snow can occur when precipitation passes through soot or petroleum-based pollutants. The snow may be oily and smell offensive. This type of snowfall is common in heavily polluted areas, or regions that have recently experienced a toxic spill or accident. Any airborne chemical can merge with snow, causing the precipitation to become colored.

Yellow snow

Avoid it. You know where it comes from!